The San Miguel plate. Relief from Copernicus GLO-30; rivers and lakes from USGS NHD; watersheds from the USGS Watershed Boundary Dataset; roads and city limits from Census TIGER 2024.
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83° on Nov. 15, 2008, the hottest reading for this week of the year since 1970. Next is 81° in 2010.
San Miguel’s center stands about 260 ft above sea level. Within three miles the land runs from 100 ft to 850 ft, a difference of 750 ft: hilly country. The highest named summit within 15 miles is Mount Diablo, about 3,810 ft, 7 miles east.
The section below crosses 11 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 6×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Sandstone, mudstone. Age: Oligocene to Pliocene, 2.6 to 34 million years old. About 5 mi of the line.
Sandstone, conglomerate. Age: Miocene to Pleistocene, less than 23 million years old. About 5 mi of the line.
Sandstone, mudstone. Age: Jurassic to Cretaceous, 66 to 202 million years old. About 3 mi of the line.
Basalt, basaltic-andesite. Age: Middle Miocene to Late Miocene, 8 to 12 million years old. About 3 mi of the line.
Siltstone-mudstone. Age: Early Cretaceous, 113 to 145 million years old. About 2 mi of the line.
Serpentinite. Age: Middle Jurassic to Late Jurassic, 145 to 175 million years old. About 2 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale. 5 smaller units along the line are not listed.
Heights from Copernicus GLO-90 (a surface model: it includes buildings and tree canopy); summits from USGS GNIS; rivers from USGS NHDPlus HR; rock from the USGS State Geologic Map Compilation.
A raindrop that lands in San Miguel reaches San Francisco Bay and the Pacific Ocean by way of Walnut Creek: about 11 river miles.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of San Miguel; river miles along the main stem, rounded.
San Miguel has a warm-summer Mediterranean climate. Summer afternoons average 75° in September; winter nights fall to about 45° in January. About 23 inches of rain fall in a normal year.
On November 14 San Miguel normally reaches 66° and falls to 50°. The record high is 82°, in 2008; the record low 37°, in 1978. The most rain on this date, 0.82 in, fell in 1972.
Normals 1991–2020 and records since 1970 at Oakland Museum, 14 miles west-southwest, 230 ft lower. NOAA, through RCC-ACIS.
